In recent years, research and development of functional materials using organic compounds have been actively attempted. In late years, development of organic electroluminescent elements (organic EL elements) using organic compounds has been vigorously advanced as constituent materials of elements.
The organic electroluminescent element is an element having a structure that a thin film containing a fluorescent organic compound is held between an anode and a cathode, in which electrons and holes are injected in the thin film for recombination to generate excitons and luminescence occurs using light released in deactivating the excitons. The organic electroluminescent element enables luminescence at a low direct current voltage of about several volts to several tens of volts. Further, luminescence of various colors (for example, red, blue and green) is possible by selecting the types of fluorescent organic compounds. The organic electroluminescent element having such characteristics is expected to be applied to various luminescent elements, display devices and the like.
Recently, due to the characteristics for enabling patterning by a printing method, as a material useful for a large screen TV panel or a flexible sheet display, development of organic electroluminescent elements using conjugated luminescent polymers (polymer organic electroluminescent elements: polymer organic EL elements) as a fluorescent organic compound has been vigorously advanced.
In the organic electroluminescent element, a hole transport layer or an electron transport layer has been used such that a luminescent layer has been held singly between electrodes or stacked thereto. However, in order to enhance characteristics or life of an element, there has been generally used a hole injection layer in the vicinity of an anode. As a material of a hole injection layer in the polymer organic electroluminescent element (hole injection material), there have been known a material of a dispersion (Bayer Co., Ltd., product name: BaytronP) containing a polythiophene derivative (poly(3,4-ethylene dioxythiophene): hereinafter referred to as PEDOT) that is an electroconductive polymer and poly(styrene sulfonic acid) (hereinafter referred to as PSS) that is a polymer protonic acid, and the like (Patent Document 1).
Patent Document 1: Japanese Patent Laid-open No. 2000-91081